DC electric heaters usually consist of a plurality of heater elements connected in parallel, series, or both. When a desirable temperature range is specified, a control system of the electric heater system controls the output power to the heater elements by turning on a determined number of heater elements while turning off the remaining heater elements to approximately meet the desired temperature. The resolution of this type of control system, however, is limited by the number of heater elements. This limitation restricts the DC electric heaters from meeting certain fine-tuned percentage output power requirements.
In order to fine-tune the output power to the heater elements, some conventional systems use Pulse Width Modulation (PWM) to control the output power to all the heater elements. One such system is described in U.S. Pat. No. 5,582,756 to Hideki Koyama. The '756 system includes a heater control device that uses a PWM signal for controlling a switch that turns on and off the entire electric heater. A PWM circuit works by making a square wave with a variable on-to-off ratio, also called a duty cycle, such that a variable amount of power is applied to the load. The duty cycle is a percentage number calculated by Ton/(Ton+Toff), where Ton is the time period when power is applied to the load, Toff is the time period when power is not applied to the load, and the duty cycle T is the total of Ton and Toff. If Ton=Toff, then the duty cycle is 50%, which means 50% of power is applied to the load. However, to achieve the desirable result, the cycle period T must be short relative to the load's response time to the change in ON/OFF state. Therefore, the PWM frequency has to be kept at a high rate. In such instances, it is not uncommon that the PWM frequency reaches tens of KHz, sometimes up to one hundred KHz or even more. As the frequency increases, the fast switching between ON and OFF states in the load circuitry will generate high input current ripple. This can affect the lifetime of certain circuitry, such as a bus capacitor, and may also cause radio frequency interference (RFI) that affects other electronic components in the DC electric heater or other nearby electronic equipment.
To address the high input current ripple problem, conventional DC electric heater systems may use additional input filters. This solution, however, will inevitably add more complexities to the circuitry and extra cost to the overall system.
Methods and systems consistent with certain features of the disclosed specification are directed to solving one or more of the problems set forth above.